It is required in the fields of biochemistry and medical measurements to measure low molecular-weight molecules such as glucose, ions, cell communication molecules, and peptides, and high molecular-weight molecules such as hormones, proteins and DNAs. In the biochemical measurements as described above, generally a quantity of samples is small, and a method is required for obtaining as much information as possible with a possibly minimum quantity of samples because minimally invasive checking is required.
As a means for measurement, a method is utilized in which a pair of molecules specifically binding to each other such as a pair of an antigen and an antibody, a pair of DNS sequences complimentary to each other, and a pair of a ligand and a receptor are used and one of the molecules in each pair is detected. In most of the cases, an analyte is detected by labeling a sample with a fluorescent material or a radioactive material and detecting the luminescence or the radioactivity. However, the method for using a label has the following problems relating to a process of labeling an analyte: (1) a sample is required to be diluted; (2) it takes time; (3) an activity of an analyte changes; and (4) specificity of an analyte changes. Therefore, a method for using a label has been sought. As described above, the biochemical measurement requires a method in which a sample required for measurement is minimal and labeling is not required.
Examples of the prior art-based technique for biochemical measurement utilizing specific binding of molecules and not requiring labeling include the method utilizing the surface plasmon resonance as disclosed, for instance, in “SPR DETECTION SYSTEM MEDIUM A” (online), Basic Principle of Technology, disclosed on a website of Biacore (searched on Jun. 13, 2005), <http://www.biacore, co.jp/3—1—3.shtml> searched on Jun. 13, 2006). A principle of this method is illustrated in FIGS. 1A to 1C. In the method cited above, as illustrated in FIG. 1A, one of a pair of molecules specifically binding to each other is fixed to an Au film on a surface of a sensor chip in advance. When an analyte is allowed to flow into a flow cell, the measuring objective molecules bond with the fixed molecules as illustrated in FIG. 1A. When such binding occurs, a refractive factor becomes larger locally at a place where the binding occurs. The change of the refractive index is detected through the surface plasmon. When light is directed to a face of the Au thin film opposite to the face to which the one of the molecules is fixed, the surface plasmon is excited under specific conditions. However, under the condition allowing for excitation of the surface plasmon, intensity of reflected light becomes lower. When a refractive index changes, the conditions allowing for excitation of the surface plasmon also change, and an angle at which intensity of the reflected light becomes lower change as shown in FIG. 1B. (The change is illustrated with the reflection light I and II in FIG. 1B). FIG. 1C illustrates that a change of the reflection light can be detected as a signal indicating a change of time during which the sample is allowed to flow in a flow cell. Therefore, by measuring angular distributions of intensities of the reflection light, a change of the refractive factor on the surface, namely, binding between one of the pair of the molecules specifically binding with the analyte in the sample can be detected. The principle of this method is based on detection of binding between biological molecules with high sensitivity by detecting a change of a refractive index on a surface of a sample via the surface plasmon phenomenon. However, the method of measuring a change of a refractive index by making use of the surface plasmon phenomenon has some problems in that the optical system inevitably becomes relatively larger with the cost high, in that the size reduction is difficult, and in that there is a limit in reducing a quantity of a sample to be measured.
A method using a photonic crystal has been studied on a technique enabling measurement of a refractive index with a minute amount of a sample. Examples of a method for measuring a refractive index using a photonic crystal include, for instance, a method described in OPTICS LETTER, Vol. 29, page 1093. A principle of the refractive index measurement using a photonic crystal described in the cited reference is now described below. The photonic crystal is a multi-dimensional periodic structure combining two or more mediums with different refractive indexes at a period of wavelength order. In the photonic crystal as described above, there is a wavelength range where light cannot propagate in the photonic crystal, namely, a frequency band called a photonic band gap. For instance, when light having a wavelength corresponding to the band gap is directed from outside to a photonic crystal, the light is completely reflected on the surface of the crystal because the light cannot be propagated inside of the crystal.
FIG. 2 illustrates a state in which a two-dimensional photonic crystal with a band gap is configured by piercing round holes in a shape of a triangular lattice on an SOI (silicon on insulator of a SiO2 substrate), and light is confined when a point defect, namely a non uniformity defect is provided in the periodic structure. Since the periodic structure is disturbed at a point defect, even light having a wavelength in the band gap can be present. However, since there is not defect around the point defect in the photonic crystal, the light can not propagate to outside, and is reflected and confined within the point defect. That is to say, the photonic crystal at and around a point defect forms a microcavity, and light having a specific wavelength is firmly confined therein in the steady state (referred to as resonant mode).
When light is introduced into the photonic crystal microcavity as illustrated in FIG. 2, only light having a wavelength corresponding to the resonant mode passes through the resonator to form a sharp peak as shown in FIG. 3. In other words, only the light having a specific wavelength passes through the resonator and the light having other wavelengths is reflected. The wavelength at the resonant peak varies depending on, for instance, a refractive index of a substance forming the photonic crystal at and around the point defect.
FIG. 4 illustrates a spectrum described in OPTICS LETTER, vol. 29, page 1093. FIG. 4 shows changes of spectrum, in a case where liquid is injected into a round hole on a two-dimensional photonic crystal with a point defect as shown in FIG. 2, when a refractive index n of the liquid is changed to 1.446, 1.448, 1.450, 1.452, and 1.454. As illustrated in FIG. 4, a peak of the spectrum changes in correspondence to a very small change of the refractive index of the liquid, and it is understood that the refractive index can be detected by measuring the peak wavelength. In this figure, the two dimensional photonic crystal is shown by way of example. However, the same effect can be obtained also by using a one-dimensional photonic crystal having a structure in which two different layers with different refractive indexes are superimposed alternately, or a three-dimensional photonic crystal having a structure in which a periodic structure is three-dimensional if liquid serving as a sample can be introduced into the photonic crystal structure.
It is possible to build a resonator without using a photonic crystal. A photonic crystal resonator has the feature in which a size of the resonator is as very small as of wavelength order. Therefore, it is possible to detect a refractive index with a minute quantity of a sample. As described above, the photonic crystal microcavity allows for use of a minimal quantity of a sample for biochemical measurement. In addition, a detector having a micro detection area provides the possibility of integration of sensors and measurement at an atomic size level.